Olefin compounds, such as ethylene and propylene, are important raw materials in the current petrochemical industry. They are largely obtained from crude petroleum by high temperature thermal decomposition and purification processes. Paraffin compounds, such as ethane and propane are also produced by the thermal decomposition steps. As a result, olefin production usually produces a mixture of olefins and paraffins. A separation process of mixtures of olefins and paraffins is very frequently needed to obtain olefin purity suitable as raw material in subsequent end use processing. The separation of ethylene from mixtures with ethane is an exemplary process of extreme industrial importance that continues to this day.
Traditionally, fractional distillation has been used for such separations and the associated unit operations have many drawbacks, such as high energy consumption, large equipment costs, labor costs, safety concerns and the like. For example, to separate ethylene/ethane mixture, a typical distillation process reportedly can involve using a distillation column incorporating contact area equivalent to least 100 theoretical trays operating at a high pressure of about 23 atm and sub-zero degree temperature. In the separation of propylene/propane, similar operation conditions at higher temperatures are also required.
Membrane technology has been considered to replace distillation for olefin-paraffin separation. Commercially practical separation of many important olefin/paraffin mixtures, for example ethylene/ethane, has been difficult to accomplish using selectively permeable polymeric membranes. Traditional polymeric membranes cannot discriminate well between ethylene and ethane with commercially attractive productivity because these compounds are similar in both molecular size and physical properties that affect selective permeability.
Facilitated transport membrane separation (“FTMS”) has arisen as an effective type of membrane process to separate olefins from paraffins. Mass transfer via FTMS is accomplished by traditional solution diffusion coupled with a selectivity-enhancing carrier mechanism. In early-developed, liquid state forms of FTMS, the carrier is in a liquid on the surface or in the pores of a membrane serving to maintain the liquid carrier adjacent to the feed side or immobilized within the membrane. To transfer across the membrane, a component of the feed associates with the carrier to become a part of the liquid carrier phase. The component and carrier travel through the membrane as a unit under an appropriate driving force. They separate on the far side discharging desired feed component(s) into the permeate stream for an end use purpose.
For separations of olefin/paraffin mixtures, a typical liquid carrier is an aqueous solution containing a metal salt. Olefin components associate preferentially with the carrier by reversibly complexing with the metal of the salt. A complexed olefin can transfer across the liquid filled membrane with significantly higher selectivity relative to the undesired paraffin feed components, than can be achieved by the non-metal-complexed olefin.
Liquid state FTMS functions for olefin/paraffin separations to an extent but has drawbacks. A principal flaw is gradual depletion of carrier that causes permeance to diminish with service time. Also, solvent or other moieties present in the liquid carrier that escape into the permeate stream need to be removed from the desired product components.
Among various techniques that have been reported to improve upon functionality of liquid state FTMS is a solid state membrane process. Some developments in this area are summarized as follows.
In U.S. Pat. No. 5,015,268 to Ho a solid homogeneous FTMS membrane is prepared from a hydrophilic polymer such as polyvinyl alcohol. The membrane does not have a liquid carrier. A metal ion or metal salt capable of complexing with aliphatically unsaturated hydrocarbons, for example silver nitrate, is distributed homogeneously in the hydrophilic polymer. Preferably the polymer is crosslinked.
Kimura et al. in U.S. Pat. No. 4,318,714 discloses FTMS using a polymeric ion exchange membrane in which the polymer has electrostatically retained counter ions reversibly reactive to the gas molecules being separated. Example 4 describes high selectivity by pure component separation of ethylene and ethane by sulfonated polyxylylene oxide ion exchange resin that was immersed in AgNO3 solution then rinsed in distilled water to remove residual AgNO3 solution and Ag+ ions. The feed gases were humidified to 90% relative humidity.
U.S. Pat. No. 4,614,524 to Kraus used a membrane of halogenated olefin polymer with pendant acid groups (i.e., Nafion® 415 ion exchange resin) that was equilibrated with AgNO3 to obtain selectivity between ethylene and ethane. Kraus teaches that the membrane must be plasticized with a polyhydric alcohol to effect separation.
Eriksen et al. U.S. Pat. No. 5,191,151 discloses a process for separating C2-C4 olefins from C1-C6 paraffins using a membrane of an ion exchange resin of tetrafluoroethylene/perfluorovinylether sulfonated copolymer with Ag+ ion exchange. The membrane is prepared by certain specific ion exchange methods that includes steps of converting the poly(perfluorosulfonic acid) from protonic form to anionic —SO3− form by contacting with an alkali metal solution, swelling the converted membrane in an alcohol, and exchanging a silver ion in the ionomer. For the membrane to separate olefin from paraffin, the gas mixture feed must be humidified.
The development of a facilitated transport separation membrane that provides high selectivity and permeance in separating olefins from gas mixtures with paraffins is greatly desired. It is further desired to have such a membrane that functions in olefin/paraffin separation with little or no limitation of humidification of the feed or plasticization of the membrane. There is much need for an olefin/paraffin membrane separation process that maintains high selectivity and permeance for extended durations. A method of manufacturing consistently high quality facilitated transport separation membranes effective to separate olefins from mixtures with paraffins with both high selectivity and permeance is also much wanted.